The present invention relates in general to information terminal systems, and more particularly to a microcomputer terminal system.
Heretofore, microcomputer terminal systems employed microprocessors. The microprocessor did not have its own memory, and had to either wait for the completion of the video refresh cycle or had to interrupt the video refresh cycle. When the microprocessor did not wait for the completion of the video refresh cycle, it interrupted the video display on the cathode ray tube. By failing to have its own memory, the microprocessor had to employ the system bus to read stored data. Thus, each time the microprocessor had to read stored data, it had to interrupt other operations in the system operating from the system bus. Therefore, only one operation could take place at a given time.
In the previously known devices, the cursor was controlled by the microprocessor and the keyboard for placement in the memory for the input/output devices. The microprocessor did not have exclusive control over the placement of the cursor in the memory for the input/output devices. Thus, the microprocessor did not know the location of the cursor at all times. This resulted in more complicated programming procedures.
When the keyboard wrote the cursor location along with the microprocessor, it required microprocessor intervention to get characters from the keyboard to the memory for the input/output devices. Accordingly, the keyboard, in the previously known devices, did not have direct access to the memory for the input/output devices. Consequently, the versatility of the microcomputer system was unduly limited. A patent of interest for digital storage and generation of video signals is the patent to Cole et al., U.S. Pat. No. 3,345,458.